Selecting By Color Channels

We can turn a channel into a selection

We can turn a selection into a channel

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What is a channel?

In this topic, we will be discussing two types of channels: color channels and alpha channels.  Specifically, we will discuss how to use a color channel to create a selection and how to store a selection as an alpha channel.  Color channels contain the color and tonal information for the colors in an image's color model.  You can read more on the Channels and Bit Depth page.  Alpha channels, as discussed on this web site, are used to store selection and layer mask information.  You can read about them on the Alpha Channels page.

In Photoshop, both alpha and color channels are displayed in the Channels palette.  An RGB image, as shown in Figure 1, will have three individual color channels and a composite channel.  The composite channel is all the individual color channels blended together.  The individual channels physically reside in your image file.  The composite channel does not.  It is a virtual rendering of your image.  A CMYK image will have four individual channels and the composite.  A grayscale image will have only one channel.

An individual color channel is represented as a black and white mask. This mask indicates how much of the channel information should be used when rendering a color. White areas in the mask use all the information.  Black areas use none.  Gray areas use some of the information.

Like layer masks, channel masks are really 'hard coded' selections.  This topic describes how to turn this hard coded selection into an active selection.  Once it is an active selection, you can use it like any active selection created by the marquee, lasso or magic wand tools.  In the example below, we will turn the active selection into a layer mask on an adjustment layer.  Keep in mind the channel itself is not deleted or altered in any way.  We are just taking the information in the channel and using it to create an active selection.
 

How A Mask Works

Lets review how a mask works as a selection.  White in a mask means the area is fully selected.  Black means the area is not selected.  Gray means it is partially selected.  The lighter the gray the more the area is selected.  The darker the gray the less the area is selected.

When using a mask with an adjustment layer, those areas of a mask that are white we be fully adjusted.  The areas that are black in the mask will not be adjusted at all.  The gray areas will be adjusted in accordance with how dark or light the gray is.  The lighter the gray, the more the area will be adjusted.
 

Turning Selections Into Channels

To learn how to turn a selection into an alpha channel, you will want to read the Savings Selections page.
 

Turning Channels Into Selections

Load An Individual Color Channel As A Selection

Follow these steps to use an individual color channel to create a selection.

1. Activate the Channels palette.
2. Click on each individual color channel until you find the one you want to use.  Make sure this is the only visible color channel.
3. Click the Load Channel As Selection icon at the bottom of the Channels palette.
4. Click on the composite channel to reactivate all color channels.
5. Click on the Layers tab to make the Layers palette the active palette.
6. You will see the active selection in the document window.

 

Load The Composite Color Channel As A Selection

To load the composite channel as a selection is easy. With the Layers palette active, press Ctrl + Alt + ~ (Command + Option + ~) (~ is the tilde character). Now that the easy part is done, lets discuss what is happening.  When you first use Ctrl + Alt + ~ (Command + Option + ~) on your image, you may think a selection is being placed around individual colors. Actually, it is selecting tonal values.  If the tone is solid white, it is fully selected.  If the tone is solid black, it is not selected.  Tones in-between are partially selected.  Light tones will be selected more than mid tones, which will be selected more than dark tones.

My experience shows that it is the perceived luminance (or %K) that is being selected.  To learn more, read the Perceived and Measured Luminance page.
 

Example

Now that we have learned how to load a channel as a selection, lets use it in a practical example.

In this example, we will use a previously color corrected image to isolate a hard to select area and change its color.  Specifically, we will isolate the yellow leaves in Figure 2A and make them orange, as seen in Figure 2B.  We used this same image on the Hue/Saturation page to also change the leaves to orange.  However, in that example we used color ranges to identify and change the colors.  In this example, we will use an individual color channel to create a mask on a Hue/Saturation adjustment layer.

Figure 2A.  Before...

Figure 2B.  and After

Below are the steps used to isolate the yellow leaves using a color channel, use the selection as a layer mask in a Hue/Saturation adjustment layer, modify the mask to our needs and change the color to orange.

1. Click on the Channels tab to make the Channels palette the active palette.
2. Click on each individual color channel and find the channel where the leaves are the whitest and easily distinguishable from their surroundings.
3. After looking at each individual color channel, the red channel best met the criteria in step 2 for this image.  Therefore, the Red channel was made the only active channel.
4. At the bottom of the Channels palette, click the Load Channel As Selection icon .
5. Click the composite channel to reactivate all channels.
6. Click the Layers tab to exit the Channels palette and activate the Layers palette.
7. Click Layer > New Adjustment Layer and choose Hue/Saturation.  Or, click the Create New Fill or Adjustment Layer icon at the bottom of the Layers palette.
8. Name the layer and click OK.  At this point, Photoshop will add the adjustment layer to the document and create a layer mask based on the selection.  The Hue/Saturation dialog box will be open and the active selection will be gone.
9. Move the Hue slider until the desired result is achieved.  In this case, Hue was set to -15.  (This means we took the existing color and replaced it with the color that is 15° counterclockwise from it on the color wheel.)  At this point, do not be concerned if other parts of the image are also changed.
10. Click OK to close the dialog box.
11. To isolate the change to the leaves, we will need to paint in the layer mask.  Alt - click the layer mask on the Hue/Saturation adjustment layer.  The layer mask will be loaded into the document window.  It should look identical to the red channel mask, as seen in Figure 3A.
12. Using black paint , the Brush tool and a soft edged brush, paint in the layer mask to cover up all areas except the leaves.  The result can be seen in Figure 3B.  (You can download both soft and hard edged brushes from the Downloads page.)
13. Alt - click the layer mask again to restore the image.  The result is shown in Figure 2B.

Figure 3A.  The Red Channel Mask

Figure 3B.  Red channel loaded into layer mask and painted black

Key Points

The key points when using this technique are as follows.

• Use the channel where the target area is the whitest and easily distinguishable from its surroundings.
• After the selection is used to create the layer mask, use black paint and the Brush tool to paint in the layer mask those areas you do not want affected by the adjustment.